Variable speed belt drives are being considered more seriously for new applications in the vehicle industry because a new generation of belts is available. These belts are rated for higher power and so can be used in a belt driven transmission for an automobile, snowmobile or other vehicle as the main transmission. Because these belt drives are infinitely variable within a particular range they can make an ideal transmission.
Although the newest generation of belts can handle surprising speeds and power, there can be a substantial amount of heat generated by the contact of the belts with the pulley discs when the variable speed belt drive experiences large variations in torque under dynamically changing conditions. In order to minimize belt slip, for example, relatively high contact pressures between the sides of the belt and the pulley discs are often resorted to on an almost continuous basis. But this undesirably increases belt tension and lowers the service life thereof.
Variable speed belt drives for steering a vehicle have also been proposed, but most of these have incorporated separate drive belts and relatively unsophisticated and/or impractical adjustable belt drive mechanisms for each side of the vehicle. U.S. Pat. No. 1,483,959 issued to L. C. Welch on Feb. 19, 1924 appears to be an exception to such duplicated and costly drive belt mechanisms by disclosing a system for steering a vehicle utilizing a single belt for transferring power between the axle shafts extending from the opposite sides of a standard differential gear assembly. Unfortunately, the shallow-angle, adjustable belt drive mechanism shown in that patent makes no provision for clamping the belt as a function of torque or for accommodating changes in the effective center distance of the belt, and has undesirable interrupted surfaces of the pulley discs. As a result, it is theorized that such mechanism would be inefficient and generally impractical for the intended purpose.
The belt drive industry has long recognized the need for automatically adjusting the clamping forces imposed on the belt by the pulley discs and/or for adjusting the belt tension as a function of the load being transmitted by the belt. For example, reference is made to the torque sensing belt drives shown in SAE Paper No. 720708 authored by L. R. Oliver and D. D. Henderson on or about Sept. 11-14, 1972 and in SAE Paper No. 720709 authored by D. L. Keller and R. E. Wilson on or about that same date. One of the pulley assemblies in these drives is equipped with a belt tensioning device including a helical cam incorporated into the axially sliding flange or pulley disc, a cooperating follower attached to the other flange, and a relatively light compression spring. With high loads belt tension is increased to avoid slippage, and with low loads belt tension is reduced for maximizing belt life. Unfortunately, these drives are limited to a single direction of rotation of the input. Another mechanism is disclosed in U.S. Pat. No. 3,625,079 issued to S. J. Hoff on Dec. 7, 1971. That patent has a normally open, speed-responsive driver pulley assembly and a normally closed, torque-responsive driven pulley utilizing a helical cam and a torsion spring. That drive mechanism, for a mini bike, is undesirably unidirectional and is not directly responsive primarily to the torque level being transmitted by the belt because of the speed responsive portion.
U.S. Pat. No. 4,173,155 issued to P. G. Togami et al on Nov. 6, 1979 shows driving and driven pulley assemblies for driving a rotor shaft in a single direction of rotation and included a torque sensing mechanical cam. The movable pulley disc associated with the cam undesirably can rotate relative to the fixed pulley disc so that only half of the total torque load on the belt passes through the cam.
U.S Pat. Nos. 4,541,820 and 4,541,821 issued to S. Sakakibara on Sept. 17, 1985 illustrate several V-belt and pulley type transmissions using a cam mechanism for urging one of the movable pulley discs toward one of the stationary pulley discs and clamping the belt as a function of torque. However, these transmissions function unidirectionally and undesirably incorporate various servo-mechanisms which add excessive control complexity and expense.
Accordingly, what is desired is an efficient, rugged and low cost adjustable belt drive mechanism for controllably transferring power between two variable pulley assemblies in either direction of rotation, for coordinating the movement of the movable pulley discs, and for simultaneously clamping the belt as a function of the load on the belt. The drive mechanism should include a simple and reliable hydraulically actuated device for axially positioning at least one of the movable pulley discs in order to change the reduction ratio thereof, and an effective mechanical means for adjusting the tension of the belt as a function of torque in order to avoid slip and to extend the service life of the belt. Moreover, the mechanism should be so constructed and arranged as to allow the elements of the pulley assemblies to rotate freely, to slide easily in the axial direction, and to transmit relatively high loads while exhibiting a minimum of distortion or tendency to cock. Preferably, the belt drive mechanism should be specifically constructed to bidirectionally distribute substantial amounts of power between the axle shafts of a vehicle such as a track-type tractor for steering purposes and to have an extended service life under adverse field conditions.